Implementing Network
Stack IPv4 and IPv6

The Network Stack IPv4
and IPv6 features are used to configure and monitor Internet Protocol Version 4
(IPv4) and Internet Protocol Version 6 (IPv6).

This module describes
the new and revised tasks you need to implement Network Stack IPv4 and IPv6 on
your
Cisco IOS XR network.

Note

For a complete
description of the Network Stack IPv4 and IPv6 commands, refer to the
Network Stack
IPv4 and IPv6 Commands module of the
Cisco ASR 9000 Series
Aggregation Services Router IP Addresses and Services Command
Reference.
To locate documentation for
other commands that appear in this chapter, use theCisco ASR 9000 Series Aggregation Services Router
Commands Master List, or search online.

Prerequisites for Implementing Network Stack IPv4 and IPv6

You must be in a user group associated with a task group that
includes the proper task IDs. The command reference guides include the task IDs
required for each command. If you suspect user group assignment is preventing
you from using a command, contact your AAA administrator for assistance.

Restrictions for Implementing Network Stack IPv4 and IPv6

In any Cisco IOS XR software release with IPv6 support, multiple IPv6 global addresses can be configured on an interface. However, multiple IPv6 link-local addresses on an interface are not supported.

Information About Implementing Network Stack IPv4 and IPv6

To implement Network Stack IPv4 and IPv6, you need to understand the following concepts:

Network Stack IPv4
and IPv6 Exceptions

The Network Stack
feature in the
Cisco IOS XR software has the following exceptions:

In
Cisco IOS XR software, the
clear ipv6
neighbors and
show ipv6
neighbors
commands include the
locationnode-id keyword. If a location is specified, only the neighbor
entries in the specified location are displayed.

The
ipv6 nd
scavenge-timeout command sets the lifetime for neighbor entries in the stale
state. When the scavenge-timer for a neighbor entry expires, the entry is
cleared.

In
Cisco IOS XR software, the
show ipv4
interface and
show ipv6
interface
commands include the
locationnode-id keyword. If a location is specified, only the interface
entries in the specified location are displayed.

Cisco IOS XR software allows conflicting IP address entries at the time of
configuration. If an IP address conflict exists between two interfaces that are
active, Cisco IOS XR software brings down the interface according to the
configured conflict policy, the default policy being to bring down the higher
interface instance. For example, if
GigabitEthernet 0/1/0/1 conflicts with
GigabitEthernet 0/2/0/1, then the IPv4 protocol on
GigabitEthernet 0/2/0/1 is brought down and IPv4 remains active
on
GigabitEthernet 0/1/0/1.

IPv4 and IPv6
Functionality

When
Cisco IOS XR software is configured with both an IPv4 and an IPv6 address, the interface
can send and receive data on both IPv4 and IPv6 networks.

The architecture of
IPv6 has been designed to allow existing IPv4 users to make the transition
easily to IPv6 while providing services such as end-to-end security, quality of
service (QoS), and globally unique addresses. The larger IPv6 address space
allows networks to scale and provide global reachability. The simplified IPv6
packet header format handles packets more efficiently. IPv6 prefix aggregation,
simplified network renumbering, and IPv6 site multihoming capabilities provide
an IPv6 addressing hierarchy that allows for more efficient routing. IPv6
supports widely deployed routing protocols such as
Open Shortest Path First (OSPF), and multiprotocol Border Gateway
Protocol (BGP).

The IPv6 neighbor
discovery (nd) process uses Internet Control Message Protocol (ICMP) messages
and solicited-node multicast addresses to determine the link-layer address of a
neighbor on the same network (local link), verify the reachability of a
neighbor, and keep track of neighboring routers.

IPv6 for Cisco IOS XR Software

IPv6, formerly named IPng (next generation) is the latest version of the Internet Protocol (IP). IP is a packet-based protocol used to exchange data, voice, and video traffic over digital networks. IPv6 was proposed when it became clear that the 32-bit addressing scheme of IP version 4 (IPv4) was inadequate to meet the demands of Internet growth. After extensive discussion, it was decided to base IPng on IP but add a much larger address space and improvements such as a simplified main header and extension headers. IPv6 is described initially in RFC 2460, Internet Protocol, Version 6 (IPv6) Specification issued by the Internet Engineering Task Force (IETF). Further RFCs describe the architecture and services supported by IPv6.

Larger IPv6 Address Space

The primary motivation for IPv6 is the need to meet the anticipated future demand for globally unique IP addresses. Applications such as mobile Internet-enabled devices (such as personal digital assistants [PDAs], telephones, and cars), home-area networks (HANs), and wireless data services are driving the demand for globally unique IP addresses. IPv6 quadruples the number of network address bits from 32 bits (in IPv4) to 128 bits, which provides more than enough globally unique IP addresses for every networked device on the planet. By being globally unique, IPv6 addresses inherently enable global reachability and end-to-end security for networked devices, functionality that is crucial to the applications and services that are driving the demand for the addresses. Additionally, the flexibility of the IPv6 address space reduces the need for private addresses and the use of Network Address Translation (NAT); therefore, IPv6 enables new application protocols that do not require special processing by border routers at the edge of networks.

IPv6 Address Formats

IPv6 addresses are represented as a series of 16-bit hexadecimal fields separated by colons (:) in the format: x:x:x:x:x:x:x:x. Following are two examples of IPv6 addresses:

2001:0DB8:7654:3210:FEDC:BA98:7654:3210

2001:0DB8:0:0:8:800:200C:417A

It is common for IPv6 addresses to contain successive hexadecimal fields of zeros. To make IPv6 addresses less cumbersome, two colons (::) can be used to compress successive hexadecimal fields of zeros at the beginning, middle, or end of an IPv6 address. (The colons represent successive hexadecimal fields of zeros.) Table 1 lists compressed IPv6 address formats.

A double colon may be used as part of the ipv6-address argument when consecutive 16-bit values are denoted as zero. You can configure multiple IPv6 addresses per interfaces, but only one link-local address.

Note

Two colons (::) can be used only once in an IPv6 address to represent the longest successive hexadecimal fields of zeros.

The hexadecimal letters in IPv6 addresses are not case-sensitive.

Table 1 Compressed IPv6 Address Formats

IPv6 Address Type

Preferred Format

Compressed Format

Unicast

2001:0:0:0:0DB8:800:200C:417A

1080::0DB8:800:200C:417A

Multicast

FF01:0:0:0:0:0:0:101

FF01::101

Loopback

0:0:0:0:0:0:0:1

::1

Unspecified

0:0:0:0:0:0:0:0

::

The loopback address listed in Table 1 may be used by a node to send an IPv6 packet to itself. The loopback address in IPv6 functions the same as the loopback address in IPv4 (127.0.0.1).

Note

The IPv6 loopback address cannot be assigned to a physical interface. A packet that has the IPv6 loopback address as its source or destination address must remain within the node that created the packet. IPv6 routers do not forward packets that have the IPv6 loopback address as their source or destination address.

The unspecified address listed in Table 1 indicates the absence of an IPv6 address. For example, a newly initialized node on an IPv6 network may use the unspecified address as the source address in its packets until it receives its IPv6 address.

Note

The IPv6 unspecified address cannot be assigned to an interface. The unspecified IPv6 addresses must not be used as destination addresses in IPv6 packets or the IPv6 routing header.

An IPv6 address prefix, in the format ipv6-prefix/prefix-length, can be used to represent bit-wise contiguous blocks of the entire address space. The ipv6-prefix argument must be in the form documented in RFC 2373, in which the address is specified in hexadecimal using 16-bit values between colons. The prefix length is a decimal value that indicates how many of the high-order contiguous bits of the address compose the prefix (the network portion of the address). For example, 2001:0DB8:8086:6502::/32 is a valid IPv6 prefix.

IPv6 Address Type: Unicast

An IPv6 unicast address is an identifier for a single interface, on a single node. A packet that is sent to a unicast address is delivered to the interface identified by that address. Cisco IOS XR software supports the following IPv6 unicast address types:

Aggregatable Global
Address

An aggregatable global
address is an IPv6 address from the aggregatable global unicast prefix. The
structure of aggregatable global unicast addresses enables strict aggregation
of routing prefixes that limits the number of routing table entries in the
global routing table. Aggregatable global addresses are used on links that are
aggregated upward through organizations, and eventually to the Internet service
providers (ISPs).

Aggregatable global
IPv6 addresses are defined by a global routing prefix, a subnet ID, and an
interface ID. Except for addresses that start with binary 000, all global
unicast addresses have a 64-bit interface ID. The current global unicast
address allocation uses the range of addresses that start with binary value 001
(2000::/3).
Figure 1shows the structure of an
aggregatable global address.

Figure 1. Aggregatable
Global Address Format

Addresses with a
prefix of 2000::/3 (001) through E000::/3 (111) are required to have 64-bit
interface identifiers in the extended universal identifier (EUI)-64 format. The
Internet Assigned Numbers Authority (IANA) allocates the IPv6 address space in
the range of 2000::/16 to regional registries.

The aggregatable
global address typically consists of a 48-bit global routing prefix and a
16-bit subnet ID or Site-Level Aggregator (SLA). In the IPv6 aggregatable
global unicast address format document (RFC 2374), the global routing prefix
included two other hierarchically structured fields named Top-Level Aggregator
(TLA) and Next-Level Aggregator (NLA).The IETF decided to remove the TLS and
NLA fields from the RFCs, because these fields are policy-based. Some existing
IPv6 networks deployed before the change might still be using networks based on
the older architecture.

A 16-bit subnet field
called the subnet ID could be used by individual organizations to create their
own local addressing hierarchy and to identify subnets. A subnet ID is similar
to a subnet in IPv4, except that an organization with an IPv6 subnet ID can
support up to 65,535 individual subnets.

An interface ID is
used to identify interfaces on a link. The interface ID must be unique to the
link. It may also be unique over a broader scope. In many cases, an interface
ID is the same as or based on the link-layer address of an interface. Interface
IDs used in aggregatable global unicast and other IPv6 address types must be 64
bits long and constructed in the modified EUI-64 format.

Interface IDs are
constructed in the modified EUI-64 format in one of the following ways:

For all IEEE 802
interface types (for example, Ethernet interfaces and FDDI interfaces), the
first three octets (24 bits) are taken from the Organizationally Unique
Identifier (OUI) of the 48-bit link-layer address (MAC address) of the
interface, the fourth and fifth octets (16 bits) are a fixed hexadecimal value
of FFFE, and the last three octets (24 bits) are taken from the last three
octets of the MAC address. The construction of the interface ID is completed by
setting the Universal/Local (U/L) bit—the seventh bit of the first octet—to a
value of 0 or 1. A value of 0 indicates a locally administered identifier; a
value of 1 indicates a globally unique IPv6 interface identifier.

For all other
interface types (for example, serial, loopback, ATM, Frame Relay, and tunnel
interface types—except tunnel interfaces used with IPv6 overlay tunnels), the
interface ID is constructed in the same way as the interface ID for IEEE 802
interface types; however, the first MAC address from the pool of MAC addresses
in the router is used to construct the identifier (because the interface does
not have a MAC address).

For tunnel
interface types that are used with IPv6 overlay tunnels, the interface ID is
the IPv4 address assigned to the tunnel interface with all zeros in the
high-order 32 bits of the identifier.

Note

For interfaces
using Point-to-Point Protocol (PPP), given that the interfaces at both ends of
the connection might have the same MAC address, the interface identifiers used
at both ends of the connection are negotiated (picked randomly and, if
necessary, reconstructed) until both identifiers are unique. The first MAC
address in the router is used to construct the identifier for interfaces using
PPP.

If no IEEE 802
interface types are in the router, link-local IPv6 addresses are generated on
the interfaces in the router in the following sequence:

The router is
queried for MAC addresses (from the pool of MAC addresses in the router).

If no MAC address
is available, the serial number of the Route Processor (RP) or line card (LC)
is used to form the link-local address.

Link-Local Address

A link-local address is an IPv6 unicast address that can be automatically configured on any interface using the link-local prefix FE80::/10 (1111 1110 10) and the interface identifier in the modified EUI-64 format. Link-local addresses are used in the neighbor discovery protocol and the stateless autoconfiguration process. Nodes on a local link can use link-local addresses to communicate; the nodes do not need site-local or globally unique addresses to communicate. Figure 1shows the structure of a link-local address.

IPv6 routers must not forward packets that have link-local source or destination addresses to other links.

Figure 2. Link-Local Address Format

IPv4-Compatible IPv6 Address

An IPv4-compatible IPv6 address is an IPv6 unicast address that has zeros in the high-order 96 bits of the address and an IPv4 address in the low-order 32 bits of the address. The format of an IPv4-compatible IPv6 address is 0:0:0:0:0:0:A.B.C.D or ::A.B.C.D. The entire 128-bit IPv4-compatible IPv6 address is used as the IPv6 address of a node and the IPv4 address embedded in the low-order 32 bits is used as the IPv4 address of the node. IPv4-compatible IPv6 addresses are assigned to nodes that support both the IPv4 and IPv6 protocol stacks and are used in automatic tunnels. Figure 1 shows the structure of an IPv4-compatible IPv6 address and a few acceptable formats for the address.

Figure 3. IPv4-Compatible IPv6 Address Format

Simplified IPv6 Packet Header

The basic IPv4 packet header has 12 fields with a total size of 20 octets (160 bits). The 12 fields may be followed by an Options field, which is followed by a data portion that is usually the transport-layer packet. The variable length of the Options field adds to the total size of the IPv4 packet header. The shaded fields of the IPv4 packet header are not included in the IPv6 packet header. (See Figure 1)

Figure 4. IPv4 Packet Header Format

The basic IPv6 packet header has 8 fields with a total size of 40 octets (320 bits). (See Figure 2.) Fields were removed from the IPv6 header because, in IPv6, fragmentation is not handled by routers and checksums at the network layer are not used. Instead, fragmentation in IPv6 is handled by the source of a packet and checksums at the data link layer and transport layer are used. (In IPv4, the User Datagram Protocol (UDP) transport layer uses an optional checksum. In IPv6, use of the UDP checksum is required to check the integrity of the inner packet.) Additionally, the basic IPv6 packet header and Options field are aligned to 64 bits, which can facilitate the processing of IPv6 packets.

Figure 5. IPv6 Packet Header Format

This table
lists the fields in the basic IPv6 packet header.

Table 2 Basic IPv6 Packet Header Fields

Field

Description

Version

Similar to the Version field in the IPv4 packet header, except that the field lists number 6 for IPv6 instead of number 4 for IPv4.

Traffic Class

Similar to the Type of Service field in the IPv4 packet header. The Traffic Class field tags packets with a traffic class that is used in differentiated services.

Flow Label

A new field in the IPv6 packet header. The Flow Label field tags packets with a specific flow that differentiates the packets at the network layer.

Payload Length

Similar to the Total Length field in the IPv4 packet header. The Payload Length field indicates the total length of the data portion of the packet.

Next Header

Similar to the Protocol field in the IPv4 packet header. The value of the Next Header field determines the type of information following the basic IPv6 header. The type of information following the basic IPv6 header can be a transport-layer packet, for example, a TCP or UDP packet, or an Extension Header, as shown in Figure 3.

Hop Limit

Similar to the Time to Live field in the IPv4 packet header. The value of the Hop Limit field specifies the maximum number of routers that an IPv6 packet can pass through before the packet is considered invalid. Each router decrements the value by one. Because no checksum is in the IPv6 header, the router can decrement the value without needing to recalculate the checksum, which saves processing resources.

Source Address

Similar to the Source Address field in the IPv4 packet header, except that the field contains a 128-bit source address for IPv6 instead of a 32-bit source address for IPv4.

Destination Address

Similar to the Destination Address field in the IPv4 packet header, except that the field contains a 128-bit destination address for IPv6 instead of a 32-bit destination address for IPv4.

Following the eight fields of the basic IPv6 packet header are optional extension headers and the data portion of the packet. If present, each extension header is aligned to 64 bits. There is no fixed number of extension headers in an IPv6 packet. Together, the extension headers form a chain of headers. Each extension header is identified by the Next Header field of the previous header. Typically, the final extension header has a Next Header field of a transport-layer protocol, such as TCP or UDP. Figure 3shows the IPv6 extension header format.

Figure 6. IPv6 Extension Header Format

This table lists the extension header types and their Next Header field values.

Table 3 IPv6 Extension Header Types

Header Type

Next Header Value

Description

Hop-by-hop options header

0

This header is processed by all hops in the path of a packet. When present, the hop-by-hop options header always follows immediately after the basic IPv6 packet header.

Destination options header

60

The destination options header can follow any hop-by-hop options header, in which case the destination options header is processed at the final destination and also at each visited address specified by a routing header. Alternatively, the destination options header can follow any Encapsulating Security Payload (ESP) header, in which case the destination options header is processed only at the final destination.

Routing header

43

The routing header is used for source routing.

Fragment header

44

The fragment header is used when a source must fragment a packet that is larger than the maximum transmission unit (MTU) for the path between itself and a destination. The Fragment header is used in each fragmented packet.

Authentication header

and

ESP header

51

50

The Authentication header and the ESP header are used within IP Security Protocol (IPSec) to provide authentication, integrity, and confidentiality of a packet. These headers are identical for both IPv4 and IPv6.

Upper-layer header

6 (TCP)

17 (UDP)

The upper-layer (transport) headers are the typical headers used inside a packet to transport the data. The two main transport protocols are TCP and UDP.

Mobility header

To be done by IANA

Extension headers used by mobile nodes, correspondent nodes, and home agents in all messaging related to the creation and management of bindings.

Path MTU Discovery for IPv6

As in IPv4, path MTU discovery in IPv6 allows a host to dynamically discover and adjust to differences in the MTU size of every link along a given data path. In IPv6, however, fragmentation is handled by the source of a packet when the path MTU of one link along a given data path is not large enough to accommodate the size of the packets. Having IPv6 hosts handle packet fragmentation saves IPv6 router processing resources and helps IPv6 networks run more efficiently.

Note

In IPv4, the minimum link MTU is 68 octets, which means that the MTU size of every link along a given data path must support an MTU size of at least 68 octets.

In IPv6, the minimum link MTU is 1280 octets. We recommend using an MTU value of 1500 octets for IPv6 links.

Note

Path MTU discovery is supported only for applications using the TCP transport.

IPv6 Neighbor Discovery

The IPv6 neighbor discovery process uses ICMP messages and solicited-node multicast addresses to determine the link-layer address of a neighbor on the same network (local link), verify the reachability of a neighbor, and keep track of neighboring routers.

IPv6 Neighbor Solicitation Message

A value of 135 in the Type field of the ICMP packet header identifies a neighbor solicitation message. Neighbor solicitation messages are sent on the local link when a node wants to determine the link-layer address of another node on the same local link. (See Figure 1.) When a node wants to determine the link-layer address of another node, the source address in a neighbor solicitation message is the IPv6 address of the node sending the neighbor solicitation message. The destination address in the neighbor solicitation message is the solicited-node multicast address that corresponds to the IPv6 address of the destination node. The neighbor solicitation message also includes the link-layer address of the source node.

Figure 7. IPv6 Neighbor Discovery—Neighbor Solicitation Message

After receiving the neighbor solicitation message, the destination node replies by sending a neighbor advertisement message, which has a value of 136 in the Type field of the ICMP packet header, on the local link. The source address in the neighbor advertisement message is the IPv6 address of the node (more specifically, the IPv6 address of the node interface) sending the neighbor advertisement message. The destination address in the neighbor advertisement message is the IPv6 address of the node that sent the neighbor solicitation message. The data portion of the neighbor advertisement message includes the link-layer address of the node sending the neighbor advertisement message.

After the source node receives the neighbor advertisement, the source node and destination node can communicate.

Neighbor solicitation messages are also used to verify the reachability of a neighbor after the link-layer address of a neighbor is identified. When a node wants to verifying the reachability of a neighbor, the destination address in a neighbor solicitation message is the unicast address of the neighbor.

Neighbor advertisement messages are also sent when there is a change in the link-layer address of a node on a local link. When there is such a change, the destination address for the neighbor advertisement is the all-nodes multicast address.

Neighbor solicitation messages are also used to verify the reachability of a neighbor after the link-layer address of a neighbor is identified. Neighbor unreachability detection identifies the failure of a neighbor or the failure of the forward path to the neighbor, and is used for all paths between hosts and neighboring nodes (hosts or routers). Neighbor unreachability detection is performed for neighbors to which only unicast packets are being sent and is not performed for neighbors to which multicast packets are being sent.

A neighbor is considered reachable when a positive acknowledgment is returned from the neighbor (indicating that packets previously sent to the neighbor have been received and processed). A positive acknowledgment—from an upper-layer protocol (such as TCP)—indicates that a connection is making forward progress (reaching its destination) or that a neighbor advertisement message in response to a neighbor solicitation message has been received. If packets are reaching the peer, they are also reaching the next-hop neighbor of the source. Therefore, forward progress is also a confirmation that the next-hop neighbor is reachable.

For destinations that are not on the local link, forward progress implies that the first-hop router is reachable. When acknowledgments from an upper-layer protocol are not available, a node probes the neighbor using unicast neighbor solicitation messages to verify that the forward path is still working. The return of a solicited neighbor advertisement message from the neighbor is a positive acknowledgment that the forward path is still working. (Neighbor advertisement messages that have the solicited flag set to a value of 1 are sent only in response to a neighbor solicitation message.) Unsolicited messages confirm only the one-way path from the source to the destination node; solicited neighbor advertisement messages indicate that a path is working in both directions.

Note

A neighbor advertisement message that has the solicited flag set to a value of 0 must not be considered as a positive acknowledgment that the forward path is still working.

Neighbor solicitation messages are also used in the stateless autoconfiguration process to verify the uniqueness of unicast IPv6 addresses before the addresses are assigned to an interface. Duplicate address detection is performed first on a new, link-local IPv6 address before the address is assigned to an interface. (The new address remains in a tentative state while duplicate address detection is performed.) Specifically, a node sends a neighbor solicitation message with an unspecified source address and a tentative link-local address in the body of the message. If another node is already using that address, the node returns a neighbor advertisement message that contains the tentative link-local address. If another node is simultaneously verifying the uniqueness of the same address, that node also returns a neighbor solicitation message. If no neighbor advertisement messages are received in response to the neighbor solicitation message and no neighbor solicitation messages are received from other nodes that are attempting to verify the same tentative address, the node that sent the original neighbor solicitation message considers the tentative link-local address to be unique and assigns the address to the interface.

Every IPv6 unicast address (global or link-local) must be checked for uniqueness on the link; however, until the uniqueness of the link-local address is verified, duplicate address detection is not performed on any other IPv6 addresses associated with the link-local address. The Cisco implementation of duplicate address detection in the Cisco IOS XR software does not check the uniqueness of anycast or global addresses that are generated from 64-bit interface identifiers.

IPv6 Router Advertisement Message

Router advertisement (RA) messages, which have a value of 134 in the Type field of the ICMP packet header, are periodically sent out each configured interface of an IPv6 router. The router advertisement messages are sent to the all-nodes multicast address. (See Figure 1.)

Figure 8. IPv6 Neighbor Discovery—Router Advertisement Message

Router advertisement messages typically include the following information:

One or more onlink IPv6 prefixes that nodes on the local link can use to automatically configure their IPv6 addresses

Lifetime information for each prefix included in the advertisement

Sets of flags that indicate the type of autoconfiguration (stateless or statefull) that can be completed

Default router information (whether the router sending the advertisement should be used as a default router and, if so, the amount of time, in seconds, that the router should be used as a default router)

Additional information for hosts, such as the hop limit and MTU a host should use in packets that it originates

Router advertisements are also sent in response to router solicitation messages. Router solicitation messages, which have a value of 133 in the Type field of the ICMP packet header, are sent by hosts at system startup so that the host can immediately autoconfigure without needing to wait for the next scheduled router advertisement message. Given that router solicitation messages are usually sent by hosts at system startup (the host does not have a configured unicast address), the source address in router solicitation messages is usually the unspecified IPv6 address (0:0:0:0:0:0:0:0). If the host has a configured unicast address, the unicast address of the interface sending the router solicitation message is used as the source address in the message. The destination address in router solicitation messages is the all-routers multicast address with a scope of the link. When a router advertisement is sent in response to a router solicitation, the destination address in the router advertisement message is the unicast address of the source of the router solicitation message.

The following router advertisement message parameters can be configured:

The time interval between periodic router advertisement messages

The “router lifetime” value, which indicates the usefulness of a router as the default router (for use by all nodes on a given link)

The network prefixes in use on a given link

The time interval between neighbor solicitation message retransmissions (on a given link)

The amount of time a node considers a neighbor reachable (for use by all nodes on a given link)

The configured parameters are specific to an interface. The sending of router advertisement messages (with default values) is automatically enabled on Ethernet and FDDI interfaces. For other interface types, the sending of router advertisement messages must be manually configured by using the no ipv6 nd suppress-ra command in interface configuration mode. The sending of router advertisement messages can be disabled on individual interfaces by using the ipv6 nd
suppress-ra command in interface configuration mode.

Note

For stateless autoconfiguration to work properly, the advertised prefix length in router advertisement messages must always be 64 bits.

IPv6 Neighbor
Redirect Message

A value of 137 in the
Type field of the ICMP packet header identifies an IPv6 neighbor redirect
message. Routers send neighbor redirect messages to inform hosts of better
first-hop nodes on the path to a destination. (See
Figure 1.)

Figure 9. IPv6 Neighbor
Discovery—Neighbor Redirect Message

Note

A router must be
able to determine the link-local address for each of its neighboring routers to
ensure that the target address (the final destination) in a redirect message
identifies the neighbor router by its link-local address. For static routing,
the address of the next-hop router should be specified using the link-local
address of the router; for dynamic routing, all IPv6 routing protocols must
exchange the link-local addresses of neighboring routers.

After forwarding a
packet, a router should send a redirect message to the source of the packet
under the following circumstances:

The destination
address of the packet is not a multicast address.

The packet was not
addressed to the router.

The packet is
about to be sent out the interface on which it was received.

The router
determines that a better first-hop node for the packet resides on the same link
as the source of the packet.

The source address
of the packet is a global IPv6 address of a neighbor on the same link, or a
link-local address.

Use the
ipv6 icmp
error-interval
global configuration command to limit the rate at
which the router generates all IPv6 ICMP error messages, including neighbor
redirect messages, which ultimately reduces link-layer congestion.

Note

A router must not
update its routing tables after receiving a neighbor redirect message, and
hosts must not originate neighbor redirect messages.

ICMP for IPv6

Internet Control Message Protocol (ICMP) in IPv6 functions the same as ICMP in IPv4—ICMP generates error messages, such as ICMP destination unreachable messages and informational messages like ICMP echo request and reply messages. Additionally, ICMP packets in IPv6 are used in the IPv6 neighbor discovery process, path MTU discovery, and the Multicast Listener Discovery (MLD) protocol for IPv6. MLD is used by IPv6 routers to discover multicast listeners (nodes that want to receive multicast packets destined for specific multicast addresses) on directly attached links. MLD is based on version 2 of the Internet Group Management Protocol (IGMP) for IPv4.

A value of 58 in the Next Header field of the basic IPv6 packet header identifies an IPv6 ICMP packet. ICMP packets in IPv6 are like a transport-layer packet in the sense that the ICMP packet follows all the extension headers and is the last piece of information in the IPv6 packet. Within IPv6 ICMP packets, the ICMPv6 Type and ICMPv6 Code fields identify IPv6 ICMP packet specifics, such as the ICMP message type. The value in the Checksum field is derived (computed by the sender and checked by the receiver) from the fields in the IPv6 ICMP packet and the IPv6 pseudoheader. The ICMPv6 Data field contains error or diagnostic information relevant to IP packet processing.

Address Repository Manager

IPv4 and IPv6 Address Repository Manager (IPARM) enforces the uniqueness of global IP addresses configured in the system, and provides global IP address information dissemination to processes on route processors (RPs) and line cards (LCs) using the IP address consumer application program interfaces (APIs), which includes unnumbered interface information.

Address Conflict Resolution

Conflict Database

IPARM maintains a global conflict database. IP addresses that conflict with each other are maintained in lists called conflict sets. These conflict sets make up the global conflict database.

A set of IP addresses are said to be part of a conflict set if at least one prefix in the set conflicts with every other IP address belonging to the same set. For example, the following four addresses are part of a single conflict set.

address 1: 10.1.1.1/16

address 2: 10.2.1.1/16

address 3: 10.3.1.1/16

address 4: 10.4.1.1/8

When a conflicting IP address is added to a conflict set, an algorithm runs through the set to determine the highest precedence address within the set.

This conflict policy algorithm is deterministic, that is, the user can tell which addresses on the interface are enabled or disabled. The address on the interface that is enabled is declared as the highest precedence ip address for that conflict set.

The conflict policy algorithm determines the highest precedence ip address within the set.

The IP address on
GigabitEthernet 0/2/0/0 is declared as highest precedence as
per the lowest rack/slot policy and is enabled. However, because the address on
interface
GigabitEthernet 0/4/0/0 does not conflict with the current
highest precedence IP address, the address on
GigabitEthernet 0/4/0/0 is enabled as well.

Recursive Resolution
of Conflict Sets

In the example below,
the address on the interface in
GigabitEthernet 0/2/0/0 has the highest precedence because it
is the lowest rack/slot. However, now the addresses on
GigabitEthernet 0/4/0/0 and
GigabitEthernet 0/5/0/0 also do not conflict with the highest
precedence IP addresses on
GigabitEthernet 0/2/0/0. However, the addresses on
GigabitEthernet 0/4/0/0 and
GigabitEthernet 0/5/0/0 conflict with each other. The conflict
resolution software tries to keep the interface that is enabled as the one that
needs to stay enabled. If both interfaces are disabled, the software enables
the address based on the current conflict policy. Because
GigabitEthernet 0/4/0/0 is on a lower rack/slot, it is enabled.

interface
GigabitEthernet 0/2/0/0: 10.1.1.1/16

interface
GigabitEthernet 0/3/0/0: 10.1.1.2/8

interface
GigabitEthernet 0/4/0/0: 10.2.1.1/16

interface
GigabitEthernet 0/5/0/0: 10.2.1.2/16

Route-Tag Support
for Connected Routes

The Route-Tag
Support for Connected Routes feature that attaches a tag with all IPv4 and IPv6
addresses of an interface. The tag is propagated from the IPv4 and IPv6
management agents (MA) to the IPv4 and IPv6 address repository managers (ARM)
to routing protocols, thus enabling the user to control the redistribution of
connected routes by looking at the route tags, by using routing policy language
(RPL) scripts. This prevents the redistribution of some interfaces, by checking
for route tags in a route policy.

The route tag
feature is already available for static routes and connected routes
(interfaces) wherein the route tags are matched to policies and redistribution
can be prevented.

Assigning IPv4 Addresses to Network Interfaces

IPv4
Addresses

A basic and required
task for configuring IP is to assign IPv4 addresses to network interfaces.
Doing so enables the interfaces and allows communication with hosts on those
interfaces using IPv4. An IP address identifies a location to which IP
datagrams can be sent. An interface can have one primary IP address and
multiple secondary addresses. Packets generated by the software always use the
primary IPv4 address. Therefore, all networking devices on a segment should
share the same primary network number.

Associated with this
task are decisions about subnetting and masking the IP addresses. A mask
identifies the bits that denote the network number in an IP address. When you
use the mask to subnet a network, the mask is then referred to as a
subnet mask.

Note

Cisco supports
only network masks that use contiguous bits that are flush left against the
network field.

The network
mask can be a four-part dotted decimal address. For example, 255.0.0.0
indicates that each bit equal to 1 means the corresponding address bit belongs
to the network address.

The network
mask can be indicated as a slash (/) and a number- a prefix length. The prefix
length is a decimal value that indicates how many of the high-order contiguous
bits of the address comprise the prefix (the network portion of the address). A
slash must precede the decimal value, and there is no space between the IP
address and the slash.

Step 4

Use the
commit or
end command.

commit—Saves the configuration changes and remains
within the configuration session.

end—Prompts user to take one of these actions:

Yes— Saves configuration changes and exits the
configuration session.

No—Exits the configuration session without
committing the configuration changes.

Cancel—Remains in the configuration mode, without
committing the configuration changes.

Step 5

show ipv4
interface

Example:

RP/0/RSP0/CPU0:router# show ipv4 interface

(Optional)
Displays the usability status of interfaces configured for IPv4.

IPv4 Virtual
Addresses

Configuring an IPv4
virtual address enables you to access the router from a single virtual address
with a management network, without the prior knowledge of which route processor
(RP) is active. An IPv4 virtual address persists across RP failover situations.
For this to happen, the virtual IPv4 address must share a common IPv4 subnet
with a Management Ethernet interface on both RPs.

The
vrf keyword supports virtual addresses on a per-VRF basis.

The
use-as-src-addr keyword eliminates the need for configuring a loopback interface
as the source interface (that is, update source) for management applications.
When an update source is not configured, management applications allow the
transport processes (TCP, UDP, raw_ip) to select a suitable source address. The
transport processes, in turn, consult the FIB for selecting a suitable source
address. If a Management Ethernet's IP address is selected as the source
address and if the
use-as-src-addr keyword is configured, then the transport substitutes the
Management Ethernet's IP address with a relevant virtual IP address. This
functionality works across RP switchovers. If the
use-as-src-addr is not configured, then the source-address selected by
transports can change after a failover and the NMS software may not be able to
manage this situation.

Note

Protocol
configuration such as tacacs source-interface, snmp-server trap-source, ntp
source, logging source-interface do not use the virtual management IP address
as their source by default. Use the
ipv4 virtual address
use-as-src-addr command to ensure that the protocol uses the
virtual IPv4 address as its source address. Alternatively, you can also
configure a loopback address with the designated or desired IPv4 address and
set that as the source for protocols such as TACACS+ via the tacacs source-interface command.

Configuring IPv6 Addressing

This task assigns IPv6 addresses to individual router interfaces and enable the forwarding of IPv6 traffic globally on the router. By default, IPv6 addresses are not configured.

Note

The ipv6-prefix argument in the ipv6 address command must be in the form documented in RFC 2373 in which the address is specified in hexadecimal using 16-bit values between colons.

The /prefix-length argument in the ipv6 address command is a decimal value that indicates how many of the high-order contiguous bits of the address comprise the prefix (the network portion of the address) A slash must precede the decimal value.

The ipv6-address argument in the ipv6 address link-local command must be in the form documented in RFC 2373 where the address is specified in hexadecimal using 16-bit values between colons.

Assigning Multiple IP Addresses to Network Interfaces

Secondary IPv4
Addresses

The
Cisco IOS XR software supports multiple IP addresses per interface. You can specify an
unlimited number of secondary addresses. Secondary IP addresses can be used in
a variety of situations. The following are the most common applications:

There might not
be enough host addresses for a particular network segment. For example, suppose
your subnetting allows up to 254 hosts per logical subnet, but on one physical
subnet you must have 300 host addresses. Using secondary IP addresses on the
routers or access servers allows you to have two logical subnets using one
physical subnet.

Many older
networks were built using Level 2 bridges, and were not subnetted. The
judicious use of secondary addresses can aid in the transition to a subnetted,
router-based network. Routers on an older, bridged segment can easily be made
aware that many subnets are on that segment.

Two subnets of a
single network might otherwise be separated by another network. You can create
a single network from subnets that are physically separated by another network
by using a secondary address. In these instances, the first network is
extended, or layered on top of the second network. Note
that a subnet cannot appear on more than one active interface of the router at
a time.

Note

If any router on a
network segment uses a secondary IPv4 address, all other routers on that same
segment must also use a secondary address from the same network or subnet.

Caution

Inconsistent use
of secondary addresses on a network segment can quickly cause routing loops.

commit—Saves the configuration changes and remains
within the configuration session.

end—Prompts user to take one of these actions:

Yes— Saves configuration changes and exits the
configuration session.

No—Exits the configuration session without
committing the configuration changes.

Cancel—Remains in the configuration mode, without
committing the configuration changes.

Configuring IPv4 and
IPv6 Protocol Stacks

This task configures
an interface in a Cisco networking device to support both the IPv4 and IPv6
protocol stacks.

When an interface in
a Cisco networking device is configured with both an IPv4 and an IPv6 address,
the interface forwards both IPv4 and IPv6 traffic—the interface can send and
receive data on both IPv4 and IPv6 networks.

Enabling IPv4 Processing on an Unnumbered Interface

IPv4 Processing on
an Unnumbered Interface

This section
describes the process of enabling an IPv4 point-to-point interface without
assigning an explicit IP address to the interface. Whenever the unnumbered
interface generates a packet (for example, for a routing update), it uses the
address of the interface you specified as the source address of the IP packet.
It also uses the specified interface address in determining which routing
processes are sending updates over the unnumbered interface. Restrictions are
as follows:

Serial
interfaces using High-Level Data Link Control (HDLC), PPP, and Frame Relay
encapsulations can be unnumbered. Serial interfaces using Frame Relay
encapsulation can also be unnumbered, but the interface must be a
point-to-point subinterface.

You cannot use
the
ping EXEC command to determine whether the interface is up,
because the interface has no IP address. The Simple Network Management Protocol
(SNMP) can be used to remotely monitor interface status.

You cannot
support IP security options on an unnumbered interface.

If you are
configuring Intermediate System-to-Intermediate System (IS-IS) across a serial
line, you should configure the serial interfaces as unnumbered, which allows
you to conform with RFC 1195, which states that IP addresses are not required
on each interface.

SUMMARY STEPS

1.configure

2.interface
type
interface-path-id

3.ipv4
unnumbered
interface-typeinterface-instance

4. Use the
commit or
end command.

DETAILED STEPS

Command or Action

Purpose

Step 1

configure

Example:

RP/0/RSP0/CPU0:router# configure

Enters
global configuration mode.

Step 2

interface
type
interface-path-id

Example:

RP/0/RSP0/CPU0:router(config)# interface gigabitethernet 0/1/0/1

Enters interface
configuration mode.

Step 3

ipv4
unnumbered
interface-typeinterface-instance

Example:

RP/0/RSP0/CPU0:router(config-if)# ipv4 unnumbered loopback 5

Enables IPv4
processing on a point-to-point interface without assigning an explicit IPv4
address to that interface.

The
interface you specify must be the name of another interface in the router that
has an IP address, not another unnumbered interface.

The
interface you specify by the
interface-type and
interface-instance arguments must be enabled (listed as “up”
in the
show
interfaces
command display).

Step 4

Use the
commit or
end command.

commit—Saves the configuration changes and remains
within the configuration session.

end—Prompts user to take one of these actions:

Yes— Saves configuration changes and exits the
configuration session.

No—Exits the configuration session without
committing the configuration changes.

Cancel—Remains in the configuration mode, without
committing the configuration changes.

Configuring ICMP Rate Limiting

IPv4 ICMP Rate Limiting

The IPv4 ICMP rate limiting feature limits the rate that IPv4 ICMP destination unreachable
messages are generated. The Cisco IOS XR software maintains two timers: one for general destination unreachable messages and one for
DF destination unreachable messages. Both share the same time limits and defaults. If the
DF keyword is not configured, the icmp ipv4
rate-limit unreachable command sets the time values for DF destination
unreachable messages. If the DF keyword is configured, its time
values remain independent from those of general destination unreachable messages.

IPv6 ICMP Rate Limiting

The IPv6 ICMP rate limiting feature implements a token bucket algorithm for limiting the
rate at which IPv6 ICMP error messages are sent out on the network. The initial
implementation of IPv6 ICMP rate limiting defined a fixed interval between error
messages, but some applications, such as traceroute, often require replies to a group of
requests sent in rapid succession. The fixed interval between error messages is not
flexible enough to work with applications such as traceroute and can cause the
application to fail. Implementing a token bucket scheme allows a number of
tokens—representing the ability to send one error message each—to be stored in a virtual
bucket. The maximum number of tokens allowed in the bucket can be specified, and for
every error message to be sent, one token is removed from the bucket. If a series of
error messages is generated, error messages can be sent until the bucket is empty. When
the bucket is empty of tokens, IPv6 ICMP error messages are not sent until a new token
is placed in the bucket. The token bucket algorithm does not increase the average rate
limiting time interval, and it is more flexible than the fixed time interval scheme.

The DF keyword limits the rate at which ICMP
destination unreachable messages are sent when code 4 fragmentation is
needed and Data Fragmentation (DF) is set, as specified in the IP header of
the ICMP destination unreachable message.

The milliseconds argument specifies the time
period between the sending of ICMP destination unreachable messages.

or

Configures the interval and bucket size for IPv6 ICMP error messages.

The milliseconds argument specifies the interval between tokens being
added to the bucket.

The optional bucketsize argument defines the maximum number of tokens
stored in the bucket.

Step 3

Use the
commit or
end command.

commit—Saves the configuration changes and remains
within the configuration session.

end—Prompts user to take one of these actions:

Yes— Saves configuration changes and exits the
configuration session.

No—Exits the configuration session without
committing the configuration changes.

Cancel—Remains in the configuration mode, without
committing the configuration changes.

Static Policy Resolution

The static policy resolution configuration prevents new address configurations from
affecting interfaces that are currently running.

SUMMARY STEPS

1.configure

2.{ipv4 | ipv6} conflict-policystatic

3. Use the
commit or
end command.

DETAILED STEPS

Command or Action

Purpose

Step 1

configure

Example:

RP/0/RSP0/CPU0:router# configure

Enters
global configuration mode.

Step 2

{ipv4 | ipv6} conflict-policystatic

Example:

RP/0/RSP0/CPU0:router(config)# ipv4 conflict-policy static

or

RP/0/RSP0/CPU0:router(config)# ipv6 conflict-policy static

Sets the conflict policy to static, that is, prevents new interface addresses from
affecting the currently running interface.

Step 3

Use the
commit or
end command.

commit—Saves the configuration changes and remains
within the configuration session.

end—Prompts user to take one of these actions:

Yes— Saves configuration changes and exits the
configuration session.

No—Exits the configuration session without
committing the configuration changes.

Cancel—Remains in the configuration mode, without
committing the configuration changes.

Longest Prefix Address Conflict Resolution

This conflict resolution policy attempts to give highest precedence to the IP address that has the longest prefix
length.

SUMMARY STEPS

1.configure

2.{ipv4 | ipv6} conflict-policylongest-prefix

3. Use the
commit or
end command.

DETAILED STEPS

Command or Action

Purpose

Step 1

configure

Example:

RP/0/RSP0/CPU0:router# configure

Enters
global configuration mode.

Step 2

{ipv4 | ipv6} conflict-policylongest-prefix

Example:

RP/0/RSP0/CPU0:router(config)# ipv4 conflict-policy longest-prefix

or

RP/0/RSP0/CPU0:router(config)# ipv6 conflict-policy longest-prefix

Sets the conflict policy to longest prefix, that is, all addresses within the
conflict set that don’t conflict with the longest prefix address of the currently
running interface are allowed to run as well.

Step 3

Use the
commit or
end command.

commit—Saves the configuration changes and remains
within the configuration session.

end—Prompts user to take one of these actions:

Yes— Saves configuration changes and exits the
configuration session.

No—Exits the configuration session without
committing the configuration changes.

Cancel—Remains in the configuration mode, without
committing the configuration changes.

Highest IP Address Conflict Resolution

This conflict resolution policy attempts to give highest precedence to the IP address
that has the highest value.

SUMMARY STEPS

1.configure

2.{ipv4 | ipv6} conflict-policyhighest-ip

3. Use the
commit or
end command.

DETAILED STEPS

Command or Action

Purpose

Step 1

configure

Example:

RP/0/RSP0/CPU0:router# configure

Enters
global configuration mode.

Step 2

{ipv4 | ipv6} conflict-policyhighest-ip

Example:

RP/0/RSP0/CPU0:router(config)# ipv4 conflict-policy highest-ip

or

RP/0/RSP0/CPU0:router(config)# ipv6 conflict-policy highest-ip

Sets the conflict policy to the highest IP
value, that is, the IP
address with the highest value gets precedence.

Step 3

Use the
commit or
end command.

commit—Saves the configuration changes and remains
within the configuration session.

end—Prompts user to take one of these actions:

Yes— Saves configuration changes and exits the
configuration session.

No—Exits the configuration session without
committing the configuration changes.

Cancel—Remains in the configuration mode, without
committing the configuration changes.

Generic Routing Encapsulation

The Generic Routing Encapsulation (GRE) tunneling protocol provides a simple, and generic approach for transporting packets of one protocol over another protocol by means of encapsulation. The packet that needs to be transported is first encapsulated in a GRE header, which is further encapsulated in another protocol like IPv4 or IPv6; and the packet is then forwarded to the destination.

A typical GRE-encapsulated packet includes:

The delivery header

The GRE header

The payload packet

A payload packet is a packet that a system encapsulates and delivers to a destination. The payload is first encapsulated in a GRE packet. The resulting GRE packet can then be encapsulated in another outer protocol and then forwarded. This outer protocol is called the delivery protocol.

Note

When IPv4 is being carried as the GRE payload, the Protocol Type field must be set to 0x800.

IPv6 as delivery and/or payload protocol is not included in the currently deployed versions of GRE.

IPv4 Forwarding over GRE Tunnels

Packets that are tunneled over GRE tunnels enter the router as normal IP packets. The packets are forwarded (routed) using the destination address of the IP packet. In the case of Equal Cost Multi Path (ECMP) scenarios, an output interface-adjacency is selected, based on a platform-specific L3 load balance (LB) hash. Once the egress physical interface is known, the packet is sent out of that interface, after it is first encapsulated with GRE header followed by the L2 rewrite header of the physical interface. After the GRE encapsulated packet reaches the remote tunnel endpoint router, the GRE packet is decapsulated. The destination address lookup of the outer IP header (this is the same as the tunnel destination address) will find a local address (receive) entry on the ingress line card.

The first step in GRE decapsulation is to qualify the tunnel endpoint, before admitting the GRE packet into the router, based on the combination of tunnel source (the same as source IP address of outer IP header) and tunnel destination (the same as destination IP address of outer IP header). If the received packet fails tunnel admittance qualification check, the packet is dropped by the decapsulation router. On successful tunnel admittance check, the decapsulation strips the outer IP and GRE header off the packet, then starts processing the inner payload packet as a regular packet.

When a tunnel endpoint decapsulates a GRE packet, which has an IPv4 packet as the payload, the destination address in the IPv4 payload packet header is used to forward the packet, and the TTL of the payload packet is decremented. Care should be taken when forwarding such a packet. If the destination address of the payload packet is the encapsulator of the packet (that is the other end of the tunnel), looping can occur. In such a case, the packet must be discarded.

RFCs

RFCs

Title

No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

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